ASTra is an asymmetric sigmoid-type activation function, recommended for use in the final network layer only, that is designed to facilitate the classification of minority examples by allocating a larger proportion of the output range to these examples. Its form is derived from that of the Richards growth equation [1]}, used to model vegetation growth and the growth of young mammals and birds, and (assuming a conventional minority-1) is defined by
\(ASTra(x,b)=1-(1+be^{bx})^{-\frac{1}{b}}.\) 
